Conventional optical fibers are made from glass materials and widely used as optical signal-transmitting mediums for instrumentation between instruments or in an instrument, for transmission of data, for medical use, for decoration, for transmission of image, etc. However, when their diameters are not sufficiently small, their flexibilities are poor. Further, they are relatively fragile and apt to be broken by impact. Furthermore, they are heavy, because their specific gravity is comparatively large. In addition, the optical fibers themselves as well as their connectors are expensive. Due to these drawbacks, attempt has been made to replace glass materials with plastic materials. The advantages with plastic materials are numerous. For instance, the resulting optical fibers are light, tough and flexible so that their diameters and numerical apertures can be made large. Further, they can be handled with ease and can readily be connected to light emitting and/or accepting elements.
In general, a plastic optical fiber is comprised of a core made of a plastic material having good optical transmission and a cladding made of a plastic material having high transparency wherein the refractive index of the core is larger than the refractive index of the cladding. In this structure, light is transmitted by reflection at the interface between the core and the cladding. The larger the difference in refractive index between the plastic materials of the core and the cladding the better the optical transmission of the optical fiber of the plastic material having good optical transmission, the ones preferred are amorphous plastics of which examples of those preferred are polymethyl methacrylate, polystyrene, etc. (cf. Japanese Patent Publication Nos. 8978/1968 and 21660/1978).
One of the drawbacks of the plastic optical fiber is that its attenuation of light transmission is larger than the glass optical fiber. The attenuation of light transmission through the plastic optical fiber is due to the generation of radio-frequency by infrared absorption vibration between the inherently present carbon-hydrogen bonds. For instance, seven time, six time and five time overtones of the infrared absorption vibration between the carbon-hydrogen bonds of aliphatic hydrocarbons appear at 560 nm, 645 nm and 760 nm, respectively, and seven time, six time and five time overtones of the infrared absorption vibration between the carbon-hydrogen bonds of aromatic hydrocarbons appear at 530 nm, 610 nm and 710 nm, respectively. Tailings of these absorptions increase attenuation at a so-called window of loss.
In order to reduce or eliminate the carbon-hydrogen absorption due to infrared absorption, it is proposed to substitute hydrogen with deuterium. U.S. Pat. No. 4,161,500 and its corresponding Japanese Patent Kokai Publication (unexamined) No. 65555/1979, for example, disclose an optical fiber which contains a core made of deuterated polymethyl methacrylate. This optical fiber has low attenuation of light transmission over a wide range of from visible light to near infrared (cf. T. Kaino, K. Jinguji and S. Nara, Appl. Phys, Lett., 42, 567(1983)). However, the optical fiber which contains the core made of deuterated polymethyl methacrylate is highly hygroscopic. Attenuation increase due to hygroscopicity is unequivocally determined from ambient relative humidity. For example, at relative humidity of 60%, the attenuation of light transmission increase is 550 dB/Km and 450 dB/Km at wavelength of 840 nm and 746 nm, respectively. Therefore, this optical fiber cannot be used in a system utilizing a near infrared light source.
Further, an increase in temperature results in the optical transmission of the plastic optical fiber being greatly reduced. Consequently, this would lessen the reliability of the said fiber as a light signal-transmitting medium. In addition, its resistance to heat is insufficient, thus restricting its use to in transportation vehicles such as automobiles, trains, vessels, aircrafts, robots, etc. The maximum temperature which polymethyl methacrylate and polystyrene can be used is at about 80.degree. C. When used at a temperature higher than about 80.degree. C., they become deformed and their microstructures are altered; thus the function as the optical fiber is impaired. Once they are used at a temperature higher than 80.degree. C., the attenuation of light transmission is high even after cooling to room temperature, and they can only be used within a very restricted temperature range. Accordingly, a plastic optical fiber which has good heat resistance is highly desired.
The applications (U.S. patent appln. Ser. No. 504,861, now U.S. Pat. No. 4,576,438, Canadian Pat. Appln. Ser. No. 430,675 and European Pat. Appln. No. 83 10 5869.8) disclose a heat-resistant optical fiber comprising a core and a cladding, characterized in that the core comprises a core polymer comprising units of a methacrylic ester, of which the ester moiety has an alicyclic hydrocarbon group of not less than 8 carbon atoms, and the cladding comprises a transparent polymeric material having a refractive index of at least 3% smaller than that of the core polymer.
Although this plastic optical fiber has satisfactory heat resistance and flexibility, it is desirable to decrease its attenuation of light transmission over a wide range of from visible light to near infrared and to improve its humidity resistance.